1. Field of the Invention
The present invention relates to a process of producing a silicon nitride (Si.sub.3 N.sub.4) sintered body, and an apparatus for producing the same.
2. Description of Related Art
In conventional production of a silicon nitride sintered body, a sintering aid such as Y.sub.2 O.sub.3, Al.sub.2 O.sub.3, MgO, or the like, is mixed with silicon nitride powder and the mixture is formed into a shaped body. The shaped body is then sintered at a high temperature, about 1800.degree. C., in an atmosphere of a N2 gas or an inactive gas including the N2 gas to produce a shaped silicon nitride sintered body.
A conventional furnace for sintering a shaped body is shown in FIG. 1 (PRIOR ART). The conventional furnace has an adiabatic material 4, a heating source such as a heater 5, and a jig 2 for supporting the shaped body. Conventional furnace materials typically comprise carbon-containing materials.
A conventional process in which the pressure of the N.sub.2 gas atmosphere is increased to a range of from 1.5 to 50 kg/cm.sup.2 to reduce thermal decomposition of silicon nitride at a high temperature, is disclosed in Japanese Patent Post-examination Publication No. Sho-58-49509.
A process to improve the surface characteristics of a shaped silicon nitride sintered body by preventing evaporation or nitration of SiO.sub.2 which is contained in the surface of the silicon nitride powder is disclosed in Japanese Patent Post-examination Publication No. Hei-3-1270. JP '1270 discloses a process in which a CO.sub.2 gas or a CO.sub.2 gas and a CO gas are mixed with an N.sub.2 gas atmosphere, and an O.sub.2 is generated by decomposition of the CO.sub.2 gas to increase the oxygen partial pressure to thereby reduce evaporation or nitration of SiO.sub.2 contained in the surface of the silicon nitride powder.
As another process of sintering a shaped silicon nitride body wherein it is buried in silicon nitride powder or powder having the same composition as the shaped body has been tried. However, the sintering atmosphere still comes in contact with the shaped body causing the same problems described above.
In conventional processes, the surface of the shaped silicon nitride body is degenerated during sintering in the furnace by contact with the sintering gas atmosphere. As a result, the mechanical properties of the sintered body are degraded. Therefore, the degenerated layer must be removed by grinding or the like. The degree of degeneration or the thickness of the degenerated layer varies depending on the sintering batches. Further, the sintered body is difficult to be ground, and when the degenerated layer becomes thicker, its thickness varies more largely among the respective sintered bodies, so that portions to be processed must be made larger. Accordingly, the cost of producing a sintered shaped body is significantly increased and the productivity is significantly decreased.
Conventional furnaces have many problems, including uneven temperature distribution and uneven regulation of the atmospheric gas so that the size of the furnace is limited and the quantity of shaped bodies to be sintered therein is limited, which are large obstacles against improving productivity.